It is common for a tensioner such as a belt tensioner to have a means to dampen movement of the tensioner arm caused by belt tension fluctuation. The required magnitude of this damping depends on many drive factors including geometry, accessory loads, accessory inertia, engine duty cycle and others. For instance, drive systems that have higher torsional input or certain transient dynamic conditions may require higher damping to sufficiently control tensioner movement. Although higher damping is very effective at controlling arm movement, it can also be detrimental to other critical tensioner functions (e.g. slow or no response to slack belt conditions). In addition, variation or change in damping that occur as a result of manufacturing variation, operating temperature and component break-in or wear can also cause the tensioner to be unresponsive.
Accessory drive systems have benefited from the use of asymmetric damping to address this problem. An asymmetrically damped tensioner provides damping when additional belt tension is encountered, but is free to respond to slack belt conditions. Although asymmetric functionality may not be required for all other front end accessory drive tensioners, the potential for increased service life, solving other transient dynamic system problems including belt slip, or simply making the tensioner less sensitive to damping variation make it a desirable design option.
Many belt tensioner damping mechanisms that utilize frictional damping use axial forces to move components of the tensioner to create the frictional force that does the damping. These designs tend to require a means to contain the axial force and some components of the belt tensioner must be more robust to withstand the axial force over the lifetime of the tensioner.